Chlorine is commonly produced through electrolytic decomposition of sodium chloride in an electrolytic cell. An equivalent amount of sodium hydroxide and chlorine is produced in this decomposition. World-wide demand for chlorine has been increasing rapidly. However, as a consequence of satisfying this demand for chlorine, an increasing amount of sodium hydroxide is also produced that exceeds its market demand. Consequently, it would be highly desirable to convert excess sodium hydroxide into products for which a higher demand exists.
Traditionally, there has been little economic incentive for conversion of the sodium hydroxide because the cost of Na.sub.2 O as sodium hydroxide was much more expensive than Na.sub.2 O as other products, such as sodium carbonate. However, the acute disposal problems presented by the excess of sodium hydroxide has effected the price relationship between these commodities, and the price of Na.sub.2 O in sodium hydroxide has become relatively less expensive than the price of Na.sub.2 O in sodium carbonate. This excess of concentrated, commercially available sodium hydroxide solutions, with concentrations as high as 50-70%, has placed a strong impetus on the art to modify its perception. As a result, various processes have arisen to carbonate concentrated NaOH solutions with carbon dioxide (CO.sub.2), producing sodium carbonate monohydrate or anhydrous sodium carbonate. These processes, however, have not resulted in a commercially feasible conversion, despite the fact that some eliminate the need for further evaporation.
In the production of chlorine from sodium chloride, chlor-alkali electrolytic cells are often used. However, two different cell types may be used, each with differing results: the mercury cell and the diaphragm cell. The purity of the chlorine produced by either cell is the same. However, the mercury cell has the advantage of producing sodium hydroxide of a very high purity, unequaled by the product from the diaphragm cell. Further, the mercury cell produces highly concentrated solutions (50-70%). In contrast, only a dilute effluent is produced from the cathode compartment of the diaphragm cell, i.e., 100-120 g of sodium hydroxide and 140-170 g of sodium chloride per liter. This product must be evaporated to produce a marketable, concentrated 50-70% sodium hydroxide solution free of the sodium chloride.
The mercury cell is not without its flaws; there are serious economic constraints attendant to its production of chlorine. First, the installation costs of a mercury cell far exceed those of a diaphragm cell per ton of chlorine produced per day. Second, mercury cell are not very energy efficient, on the order of 50%, compared to the diaphragm cell's approximately 70% efficiency. Third, in order to achieve optimal operation of a mercury cell, the brine must be thoroughly purified to remove ions such as Ca.sup.2+, Mg.sup.2+ and SO.sub.4.sup.2-, adding still higher costs to the operation of a mercury cell. In view of the shortcomings of the mercury cell, it would be desirable to provide a process that can make use of the dilute sodium hydroxide effluent from a diaphragm cell.